III-V Material Growth on Electrochemically Porosified Ge Substrates

III-V semiconductor materials for high-efficiency multi-junction solar cells are often grown on germanium (Ge) substrates. However, apart from being considered as a rare element, Ge substrates are one of the major cost shares of a III-V multi-junction solar cell. To reduce costs and material consumption, we aim at re-usable porosified Ge substrates. Prior to the growth, the porous layers are subjected to an annealing procedure to close the wafer surface and to form a predetermined breaking area some microns below the surface. Later, the III-V epitaxial layers are mechanically lifted at the porous layer, so the substrate can be reused. Here, we demonstrate the III-V epitaxy material quality by growing Al0.5Ga0.49In0.01As/Ga0.99In0.01As double heterostructures on porous Ge substrates and characterize them in detail to understand how the porous layers affect the structural and opto-electronic properties of the III-V compounds compared to a reference grown on germanium “epi-ready” substrates. We find no significant influence of the porous Ge substrate on the layer’s composition, thickness or roughness. However, cathodoluminescence measurements reveal a defect density of 4.5×105 cm-2 in comparison with 6.8×104 cm-2 for the reference case. Those defects were identified as threading dislocations by electron channeling contrast imaging. The lifetime of minority carriers measured by time resolved photoluminescence shows no difference in the low injection regime between both samples either, indicating a high quality opto-electronic material deposited on porous Ge. These first promising results indicate a path for both: reducing costs of III-V multi-junction solar cells and a reduced germanium consumption.